Impact of $ν$SMEFT operators on low-scale leptogenesis

Abstract

We investigate the impact of higher-dimensional operators on low-scale leptogenesis (LG) via oscillations of right-handed neutrinos within the neutrino-extended Standard Model Effective Field Theory ($ν$SMEFT) and discuss the connection to neutrinoless double beta decay ($0νββ$). Focusing on a dimension-six, lepton number conserving operator, we explore how new interactions can significantly alter the production and equilibration dynamics of right-handed neutrinos. We derive the relevant quantum kinetic equations incorporating both renormalizable and non-renormalizable interactions and perform a comprehensive numerical analysis for benchmark scenarios in both the oscillatory and overdamped regimes. Our results reveal that even in the absence of explicit lepton number violation by the operator, it can enhance or suppress the baryon asymmetry of the universe (BAU) by several orders of magnitude, depending on the EFT scale. We further connect these effects to predictions for $0νββ$ decay, demonstrating that the same operator can lead to enhanced decay rates, potentially within reach of the next generation of experiments. Our findings indicate that the observation of $0νββ$ could rule out a large part of the parameter space for successful low-scale LG within the $ν$SMEFT, implying low RHN masses and low reheating temperatures.

Figures (15)

• Figure 1: Benchmark points (BPs) used in this work (see Table \ref{['tab:BPs']}). We use different shapes to show which BPs are in the oscillatory regime (■) or overdamped regime (●) of leptogenesis via oscillations (see Section \ref{['sec:scales']}). The lower gray region of the parameter space is excluded from requiring that the seesaw mechanism reproduces the active neutrino masses (see Appendix \ref{['app:yuakawa']}). In the upper gray region one cannot reproduce the correct baryon asymmetry given our approximations and assumptions in Section \ref{['sec:LG']}. Note that the allowed region agrees approximately with the state-of-the-art computations, see e.g. Klaric:2020phcHernandez:2022ivz. Furthermore, we show the expected sensitivities of SHiP (green), HL-LHC (red) and FCC-ee (blue), taken from Klaric:2020phc.
• Figure 2: The Yukawa interaction arising from the operator ${\cal O}_{6}$ denoted by the blue square.
• Figure 3: $0\nu\beta\beta$ processes generated by the operators in Eq. (\ref{['cc_opt']}). The left diagram is the standard process arising from the weak interaction (orange bullet), while the middle and right ones originate from the non-standard $\nu_R$ interaction of $(\bar{L}\nu_R)(\bar{u}_RQ)$ (blue square).
• Figure 4: BP1 (Left) contributions to the total amplitude from the standard $|C_{\rm VLL}|^2$ (pink), non-standard $|C_{\rm SLR}|^2$ (blue), and mixed terms $|C_{\rm VLL}C_{\rm SLR}|$ (orange) at $\Lambda=100$ TeV as a function of $m_4$. We take $m_5=m_4+\Delta M$ with $\Delta M$ fixed as given in Tab. \ref{['tab:BPs']}. The vertical green dash-dotted line indicates the specific value of $m_4$ used for BP1. (Right) the half-lives of $0\nu\beta\beta$ decay against $m_4$. The pink line presents the prediction from the standard interactions while the blue line includes the non-standard contributions. The excluded region is represented by the gray region, and the gray dashed line represents the expected sensitivity of $T^{0\nu}_{1/2}(^{136}{\rm Xe})=2.0\times 10^{27}~$yr at KamLAND2-Zen.
• Figure 5: BP3 (Left) contributions to the total amplitude from the standard $|C_{\rm VLL}|^2$ (pink), non-standard $|C_{\rm SLR}|^2$ (blue), and mixed terms $|C_{\rm VLL}C_{\rm SLR}|$ (orange) at $\Lambda=100$ TeV as a function of $m_4$. We take $m_5=m_4+\Delta M$ with $\Delta M$ fixed as given in Tab. \ref{['tab:BPs']}. (Right) the half-lives of $0\nu\beta\beta$ against $m_4$. The prediction from the standard interactions is drawn by the pink line, while the non-standard contributions are included in the blue line. The current experimental bound is represented by the gray region, and the dashed line corresponds to the expected sensitivity of $T^{0\nu}_{1/2}(^{136}{\rm Xe})=2.0\times 10^{27}~$yr at KamLAND2-Zen. The value of $m_4$ used for BP3 is indicated by the vertical dash-dotted line.